The present invention relates to a novel film forming process which can impart excellent formability, corrosion resistance and paintability to the surface of aluminum or aluminum alloy plates or aluminum-plated steel sheet (these being hereinafter simply referred to as aluminum plates). In more particular, the invention relates to a composite film forming process suitable for aluminum plates which are subjected to processing, such as press working and the like, and used for forming structures by bonding or assembling them with steel sheet, zinc base plated steel sheet and the like by such means of joining as adhesion, bolting and so forth.
Aluminum plates are extensively used by fabricators and assemblers, for example, in household electric appliances, automobiles, building materials, etc. Most of the aluminum plates are fabricated, assembled and thereafter painted.
In said process of forming, since aluminum plates as such have insufficient formability, lubricants represented by press oil are generally coated thereon as a remedial measure in advance to forming at the working site.
When assembling and painting are conducted after forming, it is indispensable for the process to remove residual lubricating film in advance to painting, which requires degreasing and cleaning operations.
In recent years, methods have been proposed, with the aim of simplifying process steps, reducing cost and improving working environment, which intend to omit the use of press oil in forming process by using aluminum plates of which the surface has been coated with wax-based lubricants beforehand. In such methods, however, the coated lubricant must be removed at the beginning in the process of painting subsequent to the next step of assembling. Moreover, although the working environment in pressing the aluminum plates coated with wax-based lubricants is improved to some extent as compared with that in using press oil, it cannot be regarded as satisfactory.
Accordingly, proposals have been made of functional surface treated aluminum plates having more adequate lubricity.
Prior art techniques relating to functional surface treated aluminum plates include those disclosed in (A) Japanese Patent Application Kokoku (Post-Exam. Publn.) No. 63-25032, (B) Japanese Patent Application Kokai (Laid-open (unexamined)) No 62-289275 and (C) Japanese Patent Application Kokai (Laid-open) No. 63-83172. These prior art techniques will be outlined below.
(A) relates to an aqueous composition for forming lubricating coating film containing as main components a lubricant and an organic-inorganic composite reaction product comprising a water-soluble or water-dispersible organic resin, an alkoxysilane compound and silica. Since the film of an organic-inorganic composite reaction product is poor in flexibility, even when it contains a lubricating component the film cannot follow the high speed forming and is unsatisfactory in lubricity.
(B) relates to a film comprising as main components a composite substance or mixed substance consisting of urethane resin, silicon dioxide and fluororesin. Films of such compositions, however, cannot exhibit a high lubricating property as intended by the present inventors.
(C) relates to a composition comprising a resin composition composed of an organic resin selected from epoxy resin, polyester resin and acrylic resin and a curing agent component and a lubricating substance incorporated into the resin composition. The formability attainable by the surface treatment based on the above-mentioned composition, however, is still insufficient for achieving a high degree of forming intended by the present inventors.
As outlined above, the prior art methods of surface treatment which intend to impart good form-ability, corrosion resistance and paintability to the surface of aluminum or aluminum alloy plates have been unable to satisfy the requirements for high degree of formability, corrosion resistance and paintability.
The object of the present invention is to provide, overcoming the problems mentioned above, a process for forming a functional composite film which can impart a high degree of formability, i.e. an excellent lubricity, to the surface of said aluminum plates and also is excellent in corrosion resistance, paintability and chemical resistance.